1. Field of the Invention
The present invention relates generally to acoustic structures that are used to attenuate or dampen noise that emanates from a particular source. More particularly, the present invention is directed to acoustic structures that are exposed to relatively high temperatures and the systems that are used to protect such acoustic structures from damage that might be caused by such heat exposure.
2. Description of Related Art
It is widely recognized that the best way of dealing with noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic structures are typically incorporated in the engine inlet, nacelle and combustion/exhaust structures. These acoustic structures include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, acoustic materials are added to the honeycomb structure so that the honeycomb cells are acoustically closed at the end located away from the noise being dampened and covered with a porous covering at the end located closest to the noise. The closing of the honeycomb cells with acoustic material in this manner creates an acoustic resonator that provides attenuation, damping or suppression of the noise. Acoustic septums are also usually located within the interior of the honeycomb cells in order to provide the resonator with additional noise attenuation properties.
Large jet engines include a combustion or hot section that is located centrally within the engine. The hot section produces large amounts of hot combustion gases. The hot section is surrounded by an annular passageway through which air flows at much colder temperatures. The hot sections of present day jet engines typically operate at temperatures on the order of 500° F. to 750° F. The next generation of jet engines is being designed to have hot sections that operate at higher temperatures which are expected to be as high as 900° F. The higher hot section operating temperature is necessary in order to produce lower emissions and to achieve greater fuel economy.
Acoustic structures that are located near the hot sections must be protected against the relatively high temperatures in order to avoid damage to the honeycomb and/or acoustic septums. This is a particular problem for acoustic honeycomb made from composite materials which utilize matrix resins that have maximum operating temperatures on the order of 350° F. to 500° F. depending upon the type of resin. The material used to make the acoustic septum may also be damaged when exposed directly to the heat generated by the hot section.
One current approach that is used to protect acoustic structures from heat generated by the hot section is to place an insulating structure, such as a heat blanket between the hot section and the acoustic structure being protected. The heat blanket reduces the flow of heat into the acoustic structure to provide the required thermal protection. Although heat blankets provide adequate thermal insulation, they also take up valuable space and add weight to the engine. In addition, the service life of a typical heat blanket is limited so that it must be replaced at specified time intervals. The thermal blanket must also be removed to allow inspection of underlying structures. This removal and reinstallation process is time consuming and many times results in the heat blanket being damaged. Repairing and/or replacing a damaged heat blanket can involve significant added time and costs.
Another approach used to thermally protect acoustic structures is to coat the high temperature side of the acoustic structure with high temperature silicone. Such high temperature silicone coatings provide adequate thermal protection. However, the insulating coatings must be scrapped and peeled off in order to inspect the underlying acoustic structure. This is a time consuming process that also destroys the coating. A new coating must be applied to the acoustic structure once the inspection has been completed. Application of a new silicone coating is a time consuming process that includes the additional cost of the new high temperature silicone coating material.
There presently is a need to design thermal protection systems for acoustic structures that are more efficient, smaller than and not as heavy as existing thermal protection systems. The need is especially great for acoustic structures that will be used in the next generation of large jet engines where even higher hot section operating temperatures are expected.